Interference reducing amplitude modulation detector



May 3l, 1949. D. D. GRIEG 2,471,427

INTERFERENCE REDUCING AMPLITUDE MODULATION DETECTOR Filed April 16, 1945 A TT OHNE Patented May 3 1, 1949 INTERFERENCE REDUCING AMPLITUDE MODULATION DETECTOR Donald D. Grieg, Forest Hills, N. Y., assignor to Federal Telephone and Radio Corporation, Newark, N. J., a corporation of Delaware Application April 16, 1943, Serial No. 483,366

Claims.

l This invention relates to radio translators and more particularly to the problem of limiting noise effects and other interfering and signal distorting voltage fluctuations occurring in radio transmission and reception of amplitude modulated signals.

One of the objects of my invention is to provide a method and means for greatly minimizing interference noise and other disturbances in radio transmission and reception of amplitude modulated signals.

Another object of my invention is to provide a method and means to remove noise interference and other disturbances occurring in the troughs of amplitude modulation carriers as Well as distortion effects occurring at the peaks of the amplitude modulated signals.

Still another object of the invention is to provide a method and means for translating amplitude modulated Waves for passage through intermediate stages having non-linear characteristics without objectionably distorting the Waves.

Interference limiters heretofore proposed in connection with amplitude modulated signals for reducing the effect of impulse noises and similar voltage disturbances are not very satisfactory. Such limiters are operated by removing these interference voltages occurring during modulation which exceed the peak signal modulation. The disadvantage of such method of limiting is that the removal of noise cannot be complete since by operating the limiter to remove interference peaks occurring during the troughs of modulation would remove the modulation as Well. Further, such limiting does not correct or avoid objectionable distortion due to non-linearity of amplifiers and other stages used in the transmitter or receiver, as the case may be.

According to my invention, I am able not only to remove noise interference occurring during modulation peaks but also during troughs of the modulation carrier Wave thereby effecting an overall improvement in noise reduction in radio reception of amplitude modulated signals. I am also able, according to my invention, to use amplifier and other stages having non-linear characteristics Without introducing distortion in the amplitude modulated carrier wave.

The reduction in noise interference I accomplish by limit clipping the modulated carrier between two limits selected Where the sides of the undulations of the carrier wave vary in slope in substantially linear proportion to the amplitude variations of the corresponding undulations.

(Cl. Z50-20) This produces a clipped or trapezoidal wave form, the edges of each cycle of which slope in proportion to the amplitude of the corresponding undulation of the carrier. By differentiating this wave form, alternate positive and negative pulses are produced one each for each cycle of the wave. These pulses are produced in amplitude in accordance with the steepness of the edges of each cycle of the trapezoidal Wave so that they vary in amplitude according to the initial amplitude modulation of the undulation from which the cycle was produced. It therefore follows that the alternate positive and negative pulses thus produced may be detected the same as the undulations of the carrier. By gate clipping a narrow slice from the carrier, thereby limiting substantially all of the positive and negative undulations of cycles of the modulated carrier wave toi predetermined values, substantially all of the interference fluctuations are eliminated thereby greatly minimizing noise interference.

To avoid distortion in amplifier and other stages of non-linear characteristics, I translate the carrier Wave to the trapezoidal Wave form by the limit clipping operation described above, pass the Wave thus obtained through such stages and thereafter differentiate the wave to translate it into pulses defining signal envelopes substantially identical to the signal envelopes of the carrier Wave as it Was initially modulated. Since the non-linear chara-cteristics of the intermedi-` ate stages do not in this Way affect the slope relationship of the edges of the trapezoidal Wave, such stages may now be used in accordance with my invention without introducing objectionable distortion.

For a further understanding of the invention, reference may be had to the following detailed description to be read in connection with the accompanying drawings, in which:

Fig. 1 is a block diagram of radio receiver according to my invention;

Fig, 2 is a graphical illustration of an amplitude modulated carrier;

Fig. 3 is a graphical illustration of a portion of the carrier greatly enlarged and showing the translation operations of my invention; andn Fig. 4 is a block diagram of a modined arrangement of the translating features of my invention.

Referring to Fig. 1, the radio receiver is shown provided with a receiver comprising a radio frequency stage Il] and an intermediate frequency stage or stages H. The' radio frequency stage `3. The metrica defined in Ciaim 1 wherein the first-mentioned translating operation includes clipping the wave at two limits selected at opposite sides of the axis of the wave.

4. The method defined in claim 1 wherein the 'first-mentioned translating operation includes limit clipping of the wave at levels selected near the axis of the waveand the second-mentioned translating operation includes differentiating the ,trapezoidal wave form.

5. The method defined in claim 1 wherein the trapezoidal wave form obtained by the first-mentioned translating operation is treated in at least one intermediate stage of non-linear characteristics prior to the second-mentioned translating operation.

6. The method defined in claim 1 in combination with a step for reducing the carrier wave to a low intermediate frequency before translation into trapezoidal wave form, thereby insuring welldefined corners for the trapezoidal wave.

7. A method of minimizing the effects of interference voltage fiuctuations in the reception of an amplitude modulated carrier wave comprising limit clipping the modulated carrier wave between two amplitude limits within which the sides of the undulations of the carrier wave vary in slope proportional to the amplitude variations of the corresponding undulations as they normally occur in absence of interference, thereby removing a substantial portion of the uppel1 and lower parts of the undulations together with at least the major portion of interference fluctuations occurring along the carrier, and differentating the resulting clipped portion to produce alternate positive and negative pulses one each for each edge of a wave cycle, said pulses varying in amplitude substantially identically to the initial modulation of the carrier wave.

8. A system for minimizing the effects of distortion of the signal envelopes of amplitude modulated carrier waves comprising a first means to translate the undulations of the amplitude modulated wave into trapezoidal wave form the slope of the side edges of the wave cycles of which correspond proportionately to the amplitude of the corresponding undulations of the carrier wave as they normally occur in absence of interference, means to apply a carrier wave to said first means, and a second means for translating the trapezoidal wave form into pulses which vary in amplitude according to the slopes of the side edges of the Wave cycles, said pulses defining signal envelopes substantially identical to the signal envelopes of the carrier wave as it was initially modulated.

9. The system defined in claim 8 wherein the first means includes a gate clipper arranged to limit clip the wave between two limits.

10. The system defined in claim 8 wherein the rst translating means includes a gate clipper and the second translating means includes' a differentiating circuit.

11. The system defined in claim 8, in combination with an intermediate stage through which the trapezoidal wave form is passed prior to application to said second translation means.

12. A system for minimizing the effects of interference voltage fiuctuations in the reception of amplitude modulated carrier waves comprising means for limit clipping an amplitude modulated carrier wave between two amplitude limits' where the sides of the undulations of the carrier wave vary in slope proportionately to the amplitude variations of the corresponding undulations as they normally occur in absence of' interference, thereby removing a large percentage of the upper and lower parts of the undulations of the carrier wave together with substantially all of any interference uctuations occurring along the carrier wave. means to differentiate the resulting clipped portion of the carrier wave to produce alternate positive and negative pulses one each for each wave cycle, said pulses varying in amplitude according to the slope of the corresponding edges of the carrier wave so as to define signal envelopes substantially identical to the signal envelopes of the carrier wave as it was initially modulated, and means for deriving from at least certain of said pulses the envelope signals dened thereby.

13. A method of minimizing the effects of interference distortion in the reception of amplitude modulated carrier waves comprising translating the undulations of an amplitude modulated carrier wave into trapezoidal wave form the slopes if the side edges of which correspond proportionately to the amplitude of the corresponding undulations as they normally occur in absence of interference, translating the trapezoidal wave form into pulses which vary in amplitude according to the slopes of the leading and trailing edges of the trapezoidal wave form, said pulses defining signal envelopes substantially identical to the signal envelopes of the carrier wave as it was initially modulated, and deriving from said pulses the audio envelopes dened thereby.

14. A method of signal reception comprising translating the undulations of a signal amplitude modulated wave into trapezoidal wave form, the

' slope of the side edges of which correspond proportionately to the degree of signal amplitude modulation of the corresponding undulations, and translating the trapezoidal wave form into pulses which vary in amplitude according to the slope of the side edges of the trapezoidal wave, said pulses defining signal modulation envelopes substantially identical to the signal with which the wave was initially modulated.

15. The method defined in claim 14 wherein the first mentioned translating operation includes limit clipping of the wave at levels selected near the axis of the wave.

16. The method defined in claim 14 wherein the first mentioned translating operation includes limit clipping of the wave at levels selected near the axis of the wave and the second mentioned translating operation includes differentiating the trapezoidal wave form.

17. The method defined in claim 14 in combination with a step for reducing the carrier wave to a low intermediate frequency before translating into trapezoidal wave form, thereby insuring well-defined corners for the trapezoidal wave.

18. A receiver system for signal amplitude modulated waves comprising first a means to translate the undulation of a signal amplitude modulated wave wherein the slopes of the edges of the undulation in the vicinity of the zero axis thereof is proportional to substantially the instantaneous value of the amplitude modulating signal into trapezoidal wave form. the slope of the side edges of which correspond to the slope of the sides of the undulation in the vicinity of the zero axis, and a second means for translating the trapezoidal wave form into pulses which vary in amplitude according to the slope of the side edges of the wave cycles thereof, the resulting pulses defining a signal envelope substantially identical to the signal envelope of the signal with which that Wave was initially modulated.

Z 19- The.systemeeeexdinenoelaim, @wherein theT ret. Ineens inf=1use lmie1ipping' Ineens arouleeeed,telmteehp,me wane. between twoempli.- Rude limts- 29- Av receiver ,sxaftelxlA eomprsipe means for .limit Clipping@ Signelempliude .meduleted eer- Ii'l envelope subst'aienllfyA identical with which saidwave Was'rivtially modulated., -amd Ine-args for r.deriving from atlea ,Geithain of ehe pulses-.the modulating .envelope signals ,dened thereby.

DONALD D GRIEG.-

REFERENCES GITED The following neferelncesare of :record4 inthe fue of this patent:

UNITED- STATES PATENTS Number Name Date Heisng June, 1928 Hansell Ju1y.14, 1931 Leeds Oct. 27, 1936 MCCaa. Oct. v12, 1 937 Luck 1 Apr. 5,1933 Peterson May 26,119,412 Hadfleld Jan. 19 1943 Carnahan J uly` .6, :1943 

